Lubricating oil composition for an internal combustion engine

ABSTRACT

The lubricating oil composition for an internal combustion engine of the invention contains (A) a lubricant base oil composed of a mineral oil and/or a synthetic oil, (B) a boron-containing alkenylsuccinimide and/or a boron-containing alkylsuccinimide in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on the total amount of the composition, and (C) a poly(meth)acrylate in an amount of 0.1 to 30% by mass based on the total amount of the composition, and the poly(meth)acrylate has Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, in which the weight-average molecular weight thereof is represented by Mw and the mean carbon number of the alkyl groups therein, as measured through  13 C-NMR, is represented by X.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition for an internal combustion engine.

BACKGROUND ART

On an internal combustion engine for use in automobiles and others, various demands are made for high-power performance with downsizing, energy saving, emission control and the like, and for satisfying these requirements, various additives such as an anti-wear agent, a metallic detergent, an ashless dispersant, an antioxidant, a viscosity index improver and the like are blended into an engine oil.

Heretofore, an engine oil is desired to have an improved performance from various viewpoints. For example, it is desired to prevent coking that may be caused by carbonization of an engine oil, and to prevent copper release from engine parts. For these requirements, for example, PTL 1 discloses use of a hydrazide derivative having a specific structure as an additive for preventing copper release. PTL 2 discloses combined use of a specific molybdenum-type additive and a sulfurized fatty acid ester for preventing coking.

CITATION LIST Patent Literature

PTL 1: Japanese Patent 4477337

PTL 2: JP 2005-247995A

SUMMARY OF INVENTION Technical Problem

With the advance of performance enhancement and power increase thereof, the driving condition for an internal combustion engine has become increasingly severer year by year. Accordingly, it has become necessary to further enhance the oxidation stability of an engine oil, and the requirement for preventing the reduction in the base number thereof over a long period of time has become increased more.

On the other hand, for example, it is known that, in driving in an urban district where stop-and-go is repeated, copper release often occurs. Further, recently, not only in a high-speed-driving region but also in a low-speed driving region such as driving in an urban district, the driving is sometimes done with high-power utilizing a turbo charger (supercharging), and from now, it is expected that use of an engine having a turbo mechanism mounted therein would increase. However, it has become known that coking may occur more readily in a turbo mechanism-mounting engine.

Consequently, for an engine oil, it has become necessary to prevent both coking and copper release in a well-balanced manner.

However, the formulations disclosed in PTLs 1 and 2 are techniques for separately and individually preventing coking and copper release, and therefore, it is difficult to effectively prevent both coking and copper release while base number reduction is prevented.

The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a lubricating oil composition for an internal combustion engine capable of preventing base number reduction, coking occurrence and copper release in a well-balanced manner.

Solution to Problem

The present inventors have assiduously studied and, as a result, have found that combined use of a boron-containing succinimide and a poly(meth)acrylate where the ratio of the weight-average molecular weight (Mw) of the polymer to the mean carbon number of the alkyl groups in the side chains is specifically defined can solve the above-mentioned problems, and have completed the present invention. The present invention provides the following (1) to (9).

(1) A lubricating oil composition for an internal combustion engine, containing:

(A) a lubricant base oil composed of a mineral oil and/or a synthetic oil;

(B) a boron-containing alkenylsuccinimide and/or a boron-containing alkylsuccinimide in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on the total amount of the composition; and

(C) a poly(meth)acrylate in an amount of 0.1 to 30% by mass based on the total amount of the composition, the poly(meth)acrylate having Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, in which the weight-average molecular weight thereof is represented by Mw and the mean carbon number of the alkyl groups therein, as measured through ¹³C-NMR, is represented by X.

(2) The lubricating oil composition for an internal combustion engine according to the above (1), wherein Mw/X is 30,000 to 200,000. (3) The lubricating oil composition for an internal combustion engine according to the above (1) or (2), wherein the poly(meth)acrylate (C) is a non-dispersive one. (4) The lubricating oil composition for an internal combustion engine according to any of the above (1) to (3), wherein the viscosity index of the lubricant base oil (A) is 90 or more. (5) The lubricating oil composition for an internal combustion engine according to any of the above (1) to (4), wherein the mineral oil has the paraffin content (% C_(P)) according to ring analysis of 60% or more. (6) The lubricating oil composition for an internal combustion engine according to any of the above (1) to (5), which contains at least one selected from (D) a zinc dithiophosphate and (E) an alkali metal detergent or an alkaline earth metal detergent. (7) The lubricating oil composition for an internal combustion engine according to the above (6), which contains the zinc dithiophosphate (D) in an amount of 0.01 to 0.15% by mass as a phosphorus-equivalent amount, and the alkali metal detergent or the alkaline earth metal detergent (E) in an amount of 0.1 to 0.3% by mass as a metal-equivalent amount, based on the total amount of the composition. (8) The lubricating oil composition for an internal combustion engine according to any of the above (1) to (7), wherein the composition has the kinematic viscosity at 100° C. of 4 to 17 mm²/s. (9) A method of producing a lubricating oil composition for an internal combustion engine, in which the method includes:

blending (B) a boron-containing alkenylsuccinimide and/or a boron-containing alkylsuccinimide in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on the total amount of the composition, and (C) a poly(meth)acrylate in an amount of 0.1 to 30% by mass based on the total amount of the composition, into (A) a lubricant base oil composed of a mineral oil and/or a synthetic oil,

the poly(meth)acrylate (C) having Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, in which the weight-average molecular weight thereof is represented by Mw and the mean carbon number of the alkyl groups therein, as measured through ¹³C-NMR, is represented by X.

Advantageous Effects of Invention

In the present invention, there can be provided a lubricating oil composition for an internal combustion engine capable of preventing base number reduction, coking occurrence and copper release in a well-balanced manner.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention are described in detail hereinunder.

[Lubricating Oil Composition for Internal Combustion Engine]

The lubricating oil composition for an internal combustion engine (hereinafter this may be simply referred to as “lubricating oil composition”) of the present invention contains (A) a lubricant base oil, (B) a boron-containing alkenylsuccinimide and/or a boron-containing alkylsuccinimide (hereinafter these may be simply referred to as “boron-containing succinimide”), and (C) a poly(meth)acrylate. The components are described in more detail hereinunder.

[(A) Lubricant Base Oil]

The lubricant base oil (A) in the present composition is of a mineral oil and/or a synthetic oil, for which any one suitably selected from mineral oils and synthetic oils heretofore used as a base oil for lubricating oil can be used.

Examples of the mineral oil include a mineral oil refined by subjecting a lubricating oil distillate that is obtained by distilling under a reduced pressure the atmospheric residue given by atmospheric distillation of crude oil, to one or more treatments selected from solvent deasphalting, solvent extraction, hydro-cracking, solvent dewaxing, catalytic dewaxing, hydrorefining and the like, a lubricant base oil produced by isomerization of wax or GTL WAX, and the like. Among these, a mineral oil treated by hydrorefining is preferred. The mineral oil treated by hydrorefining can readily better the % C_(P) and the viscosity index to be mentioned below.

Examples of the synthetic oil include polybutene, poly-alpha-olefins such as α-olefin homopolymers and copolymers (e.g., ethylene-α-olefin copolymers), etc., various kinds of esters, for example, polyol esters, dibasic acid esters, phosphate esters, etc., various kinds of ethers, for example, polyphenyl ethers, etc., polyglycols, alkylbenzenes, alkylnaphthalenes, a lubricant base oil produced through isomerization of GTL WAX. Among those synthetic oils, poly-alpha-olefins and esters are particularly preferred and the combination of two kinds of these is also preferably used for the synthetic oil.

In the present invention, as the lubricant base oil, the above mineral oils may be used alone or in combination of two or more thereof. Alternatively, the above synthetic oils may be used alone or in combination of two or more thereof. Furthermore, one or more of the mineral oils and one or more of the synthetic oils may be used in combination thereof.

In the lubricating oil composition, the lubricant base oil (A) is to be a main component, and is contained, relative to the total amount of the lubricating oil composition, generally in an amount of 50% by mass or more, preferably 60 to 97% by mass, more preferably 65 to 95% by mass.

The viscosity of the lubricant base oil (A) is not specifically limited. Preferably, the kinematic viscosity thereof at 100° C. is within a range of 1.0 to 20 mm²/s, more preferably within 1.5 to 15 mm²/s, even more preferably within 2.0 to 13 mm²/s. In the present invention, when the kinematic viscosity of the lubricant base oil (A) is a relatively low viscosity as mentioned above, the energy-saving performance can be readily realized. In this description, the kinematic viscosity is measured according to the method described in the section of Examples to be given hereinunder.

The viscosity index of the lubricant base oil (A) is preferably 90 or more, more preferably 95 or more, even more preferably 100 or more. The upper limit of the viscosity index of the lubricant base oil is not specifically limited, but is preferably 170 or less, more preferably 160 or less, even more preferably 150 or less.

When the viscosity index of the lubricant base oil falls within the above range, the viscosity characteristics of the lubricating oil composition can be readily bettered. In this description, the viscosity index is measured according to the method described in the section of Examples to be given hereinunder.

The paraffin content according to ring analysis (% C_(P)) of the mineral oil is preferably 60% or more, more preferably 65% or more. When the paraffin content is 60% or more, the oxidation stability of the base oil can be bettered and base number reduction and coking occurrence in the lubricating oil composition can be thereby prevented. Measurement of the paraffin content (% C_(P)) will be described hereinunder.

[(B) Boron-Containing Succinimide]

The boron-containing succinimide (B) for use in the present invention includes an alkenyl or alkylsuccinic monoimide boride, and an alkenyl or alkyl succinic bisimide boride. The example of alkenyl or alkylsuccinic monoimide includes compounds represented by the following general formula (1). The example of alkenyl or alkylsuccinic bisimide includes, compounds represented by the following general formula (2). In the present invention, the good detergency of the composition is exhibited by blending the component (B). In addition, combined use with the component (C) can prevent coking occurrence and copper release.

In the above formulae (1) and (2), R¹, R³ and R⁴ each represent an alkenyl group or an alkyl group, and the weight-average molecular weight of the group is preferably 500 to 3,000, more preferably 1,000 to 3,000, respectively.

When the weight-average molecular weight of R¹, R³ and R⁴ is 500 or more, the solubility in the base oil is high and when it is 3,000 or less, the effect to be given by the compound is expected to be suitably exhibited. R³ and R⁴ may be the same or different.

R², R⁵ and R⁶ each represent an alkylene group having 2 to 5 carbon atoms, and R⁵ and R⁶ may be the same or different. m indicates an integer of 1 to 10, and n indicates 0 or an integer of 1 to 10. Here, m is preferably 2 to 5, more preferably 3 to 4. When m is 2 or more, the effect to be given by the compound is expected to be suitably exhibited. When m is 5 or less, the solubility in the base oil can be further bettered.

In the above formula (2), n is preferably 1 to 4, more preferably 2 to 3. When n is 1 or more, the effect to be given by the compound is expected to be suitably exhibited. When n is 4 or less, the solubility in the base oil can be further bettered.

The alkenyl group includes, for example, a polybutenyl group, a polyisobutenyl group, and an ethylene-propylene copolymer. The alkyl group includes ones derived from hydrogenation of those groups. As a preferred alkenyl group, there is mentioned a polybutenyl group or a polyisobutenyl group. As the polybutenyl group, a polymerized product of a mixture of 1-butene and isobutene or high-purity isobutene is favorably used. Representative examples of a preferred alkyl group include those prepared though hydrogenation of a polybutenyl group or a polyisobutenyl group.

The boron-containing succinimide (B) may be produced according to a conventionally-known method. For example, a polyolefin is reacted with a maleic anhydride to give an alkenylsuccinic anhydride, and this is further reacted with an intermediate prepared through reaction of a polyamine with a boron compound such as a boron oxide, a boron halide, a boric acid, a boric anhydride, a borate ester, an ammonium borate or the like and is imidated. The monoimide or the bisimide may be produced by varying the ratio of the alkenylsuccinic anhydride or the alkylsuccinic anhydride to the polyamine.

Alternatively, the boron-containing succinimide (B) may be obtained by treating a boron-free alkenyl or alkylsuccinic monoimide or alkenyl or alkylsuccinic bisimide with the above-mentioned boron compound.

As the olefin monomer to form the above polyolefin, usable is/are one alone or two or more of α-olefins having 2 to 8 carbon atoms, either singly or as combined. Preferred is use of a mixture of isobutene and 1-butene.

On the other hand, the polyamine includes a simple diamine such as ethylenediamine, propylenediamine, butylenediamine, and pentylenediamine; a polyalkylenepolyamine such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, di(methylethylene)triamine, clibutylenetriamine, tributylenetetramine, and pentapentylenehexamine; a piperazine derivative such as aminoethylpiperazine.

The above component (B) is contained in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on the total amount of the composition. When the content is less than 0.001% by mass, coking occurrence and copper release could hardly be prevented. When it is more than 0.1% by mass, precipitates may form and the effect that could match the incorporated amount would hardly be exhibited. From these viewpoints, the content of the component (B) is, as a boron-equivalent amount based on the total amount of the composition, more preferably 0.005 to 0.08% by mass, even more preferably 0.010 to 0.06% by mass.

The ratio by mass of boron to nitrogen (B/N ratio) in the component (B) is preferably 0.8 or more, more preferably 1.0 or more, even more preferably 1.1 or more. Though the upper limit of the B/N ratio is not specifically limited, the B/N ratio is preferably 2.0 or less, more preferably 1.5 or less, even more preferably 1.3 or less. When the B/N ratio falls within the above range, the effect to be given by the compound is expected to be suitably exhibited.

The content of the component (B) may be such that the boron-equivalent content thereof falls within the above range, and is generally 0.1 to 10% by mass or so based on the total amount of the composition, preferably 0.5 to 5% by mass, more preferably 1 to 4% by mass.

[(C) Poly(meth)acrylate]

The poly(meth)acrylate (C) to be contained in the lubricating oil composition of the present invention has Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, in which the weight-average molecular weight thereof is represented by Mw and the mean carbon number of the alkyl groups therein, as measured through ¹³C-NMR, is represented by X.

Measurement methods for Mw and X are as described in the section of Examples to be given hereinunder. The alkyl group means all the alkyl groups existing in the poly(meth)acrylate, and for example, in the general formula (3) to be mentioned below, it means R⁷ and R⁸. In the case where an alkyl group bonds to COO⁻ of the (meth)acrylate via any other substituent, such an alkyl group is also included. The mean carbon number means an arithmetic mean value.

In the present invention, the component (C) is contained in addition to the above-mentioned component (B) and therefore the lubricating oil composition can be protected from copper release and coking occurrence in a well-balanced manner. Though not clear, the principle could be presumed as follows. It is presumed that a part of poly(meth)acrylate (hereinafter this may also be referred to as “PMA”) may form a complex with copper through decomposition or the like, to thereby often cause copper release from alloys of members such as engine bearing parts, etc. When PMA has a structure capable of being entangled with each other, the amount of PMA to adhere to the metal surface of an engine and, as a result, copper release can be thereby prevented. In addition, when PMA is decomposed, its reactivity increases and owing to this, coking and copper release would be promoted. In the present invention, due to the effect of the above-mentioned component (B), entanglement of PMA is promoted while PMA decomposition is prevented, and accordingly, copper release and coking occurrence in the lubricating oil composition can be prevented in a well-balanced manner.

In the present invention, the balance between Mw and the size of the alkyl groups in the side chains of PMA is important. It is presumed that, when a large number of small alkyl groups exist in the side chains, PMA may be readily entangled even though Mw is relatively low, while on the other hand, in the case where large alkyl groups exist in the side chains in a predetermined ratio or more, PMA could hardly be entangled even though Mw is relatively high. Further, in the case where large alkyl groups exist in the side chains in a predetermined ratio or more and where Mw is relatively high, it is presumed that PMA could also be hardly entangled but PMA would readily decompose. Accordingly, when Mw/X is less than 30,000, adhesion of PMA to the metal surface of an engine could not be fully reduced but rather PMA decomposition may readily occur and, as a result, copper release and coking occurrence could hardly be prevented.

When Mw falls within a predetermined range, the reactivity of PMA could be small even though alkyl groups having certain size exist in the side chains in large numbers, but when Mw is more than 700,000, it is presumed that the reactivity of PMA increases even though a large number of small alkyl groups exist in the side chains, therefore easily causing coking and copper release. When the molecular weight is less than 100,000, it is presumed that the polymer would be hardly entangled even though many small alkyl groups exist in the side chains and copper release could not be prevented sufficiently.

When the component (C) whose Mw and Mw/X each fall within a specific range is contained, the oxidation stability is enhanced and the base number reduction can be prevented.

For preventing copper release and coking in a well-balanced manner, Mw/X is preferably 30,000 to 200,000, more preferably 30,000 to 130,000; and from the viewpoint of more suitably preventing copper release, the ratio is still more preferably 30,000 to 100,000.

The weight-average molecular weight (Mw) is preferably 100,000 to 700,000, more preferably 150,000 to 600,000, even more preferably 180,000 to 550,000.

The poly(meth)acrylate (C) is preferably a polymer of polymerizable monomers that include a (meth)acrylate monomer represented by the following general formula (3).

In the general formula (3), R⁷ represents a hydrogen atom or a methyl group, R⁸ represents a linear or branched alkyl group having 1 to 200 carbon atoms. R⁸ is preferably an alkyl group having 1 to 40 carbon atoms, more preferably an alkyl group having 1 to 28 carbon atoms, even more preferably an alkyl group having 1 to 25 carbon atoms.

In the general formula (3), specifically, examples of R⁸ include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a heneicosyl group, a docosyl group, a tricosyl group, a tetracosyl group, a pentacosyl group, a hexacosyl group, a heptacosyl group, an octacosyl group, a nonacosyl group, a triacontyl group, a hentriacontyl group, a dotriacontyl group, a tritriacontyl group, a tetracontyl group, a pentatriacontyl group, a hexatriacontyl group, an octatriacontyl, a tetracontyl group, etc., and these may be linear or branched.

In the present invention, the component (C) is preferably a non-dispersive one. As the non-dispersive poly(meth)acrylate, specifically, there is mentioned a homopolymer of one kind of a monomer represented by the general formula (3), or a (poly)methacrylate obtained through copolymerization of two or more kinds of the monomer.

However, the poly(meth)acrylate (C) may also be a dispersive poly(meth)acrylate. As the dispersive poly(meth)acrylate, there is mentioned those produced through copolymerization of a monomer represented by the general formula (3) and one or more kinds of monomers selected from the following general formulae (4) and (5).

In the general formula (4), R⁹ represents a hydrogen atom or a methyl group, R¹⁰ represents an alkylene group having 1 to 28 carbon atoms, E¹ represents an amine residue or a heterocyclic residue having 1 to 2 nitrogen atoms and 0 to 2 oxygen atoms, and a is 0 or 1.

In the general formula (5), R¹¹ represents a hydrogen atom or a methyl group, and E² represents an amine residue or a heterocyclic residue having 1 to 2 nitrogen atoms and 0 to 2 oxygen atoms.

Specifically, examples of the group represented by E¹ and E² include a dimethylamino group, a diethylamino group, a dipropylamino group, a dibutylamino group, an anilino group, a toluidino group, a xylidino group, an acetylamino group, a benzoylamino group, a morpholino group, a pyrrolyl group, a pyrrolino group, a pyridyl group, a methylpyridyl group, a pyrrolidinyl group, a piperidinyl group, a quinolyl group, a pyrrolidonyl group, a pyrrolidono group, an imidazolino group, a pyrazino group, etc.

Specifically, preferred examples of the monomer represented by the general formulae (4) and (5) include dimethylaminomethyl methacrylate, diethylaminomethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-methyl-5-vinylpyridine, morpholinomethyl methacrylate, morpholinoethyl methacrylate, N-vinylpyrrolidone and their mixtures, etc.

The copolymerization molar ratio in the copolymer of the monomer (M1) represented by the general formula (3) and the monomer (M2) represented by the general formulae (4) and/or (5) is not specifically limited. Preferably, M1/M2 molar ratio is 99/1 to 80/20 or so, more preferably 98/2 to 85/15, even more preferably 95/5 to 90/10.

In the component (C) in the present invention, the monomer represented by the general formula (3) preferably accounts for 70% by mass or more in all monomers constituting the component (C), more preferably 85% by mass or more, even more preferably 90% by mass or more.

The component (C) may contain a constituting unit derived from a monomer except those of the above general formulae (3) to (5) within a range not contradictory to the object of the present invention. In general, the monomer component of this type accounts for 10% by mass or less of all monomers.

More specifically, the above component (C) includes those produced through copolymerization of at least an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 1 to 4, and an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 12 to 40, or those produced through copolymerization of at least an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 1 to 4, an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 5 to 11, and an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 12 to 40. Among these, preferred are those produced through copolymerization of at least an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 1 to 4 and an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 12 to 40; and more preferred are those produced through copolymerization of at least methyl (meth)acrylate monomer and an alkyl (meth)acrylate monomer where the carbon number of the alkyl group is 16 to 25.

The content of the poly(meth)acrylate (C) is 0.1 to 30% by mass based on the total amount of the composition. When the content is less than 0.1% by mass, it would be difficult to prevent base number reduction, coking occurrence and copper release in a well-balanced manner. When it is more than 30% by mass, the effect balanced with the content could hardly be exhibited. The content of the above component (C) is preferably 0.3 to 25% by mass, more preferably 0.5 to 10% by mass. The content of the component (C) means the content of the resin fraction in the component.

[(D) Zinc Dithiophosphate]

The lubricating oil composition of the present invention may contain (D) a zinc dithiophosphate. The incorporation of a zinc dithiophosphate (D) betters wear-resistant properties and oxidation stability. As the zinc dithiophosphate, there are mentioned compounds represented by the following general formula (6).

In the general formula (6), R¹², R¹³, R¹⁴ and R¹⁵ each independently represent a hydrocarbon group having 1 to 24 carbon atoms. The hydrocarbon group is any of a linear or branched alkyl group having 2 to 24 carbon atoms, a linear or branched alkenyl group having 3 to 24 carbon atoms, a cycloalkyl group or a linear or branched alkylcycloalkyl group having 5 to 13 carbon atoms, an aryl group or a linear or branched alkylaryl group having 6 to 18 carbon atoms, and an arylalkyl group having 7 to 19 carbon atoms. Among these, an alkyl group is preferred.

Specifically, the zinc dithiophosphate is preferably a zinc dialkyldithiophosphate, and more preferably a zinc secondary dialkyldithiophosphate.

The content of the zinc dithiophosphate is preferably 0.005 to 0.30% by mass as a phosphorus-equivalent amount relative to the total amount of the composition, more preferably 0.01 to 0.15% by mass. Falling within the above range, the wear-resistant properties and the oxidation stability of the lubricating oil composition can be bettered without having influences on the detergency and the coking resistance.

[(E) Metallic Detergent]

The lubricating oil composition may further contain (E) a metallic detergent selected from an alkali metal detergent or an alkaline earth metal detergent. The incorporation of the metallic detergent (E) betters detergency and can readily prevent base number reduction, coking occurrence and copper release.

Specifically, there are mentioned one or more metallic detergents selected from an alkali metal sulfonate or an alkaline earth metal sulfonate, an alkali metal phenate or an alkaline earth metal phenate, an alkali metal salicylate or an alkaline earth metal salicylate, and so on. The alkali metal includes sodium and potassium, and the alkaline earth metal includes magnesium and calcium. Sodium as the alkali metal, and magnesium and calcium as the alkaline earth metals are preferably used, and calcium is more preferred.

The alkali metal detergent or the alkaline earth metal detergent may be neutral, basic or overbased, and basic or overbased ones are preferred. Preferably, those having a total base number of 10 to 500 mgKOH/g are used, and those having a total base number of 150 to 450 mgKOH/g are more preferred. The total base number is measured according to a perchloric acid method of JIS K-2501.

As the metallic detergent (E), for example, one having a total base number of 150 to 450 mgKOH/g may be used singly, or an alkali metal detergent or an alkaline earth metal detergent having a total base number of 150 to 450 mgKOH/g and an alkali metal detergent or an alkaline earth metal detergent having a total base number of 5 to 100 mgKOH/g may be used as combined.

The content of the metallic detergent (E) is preferably 0.05 to 0.5% by mass as a metal-equivalent amount relative to the total amount of the composition, more preferably 0.1 to 0.3% by mass. The incorporation of the component in an amount not lower than the lower limit can more readily prevent base number reduction, coking occurrence and copper release. The incorporation of the component in an amount not higher than the upper limit makes it possible to exhibit the effect comparable to the content thereof.

More preferably, the lubricating oil composition contains the zinc dithiophosphate (D) in an amount of 0.01 to 0.15% by mass as a phosphorus-equivalent amount and the metallic detergent (E) in an amount of 0.1 to 0.3% by mass as a metal-equivalent amount, based on the total amount of the composition.

[Other Components]

The lubricating oil composition may contain a boron-free succinimide in addition to the boron-containing succinimide (B). The boron-free succinimide is an alkenylsuccinimide and/or an alkylsuccinimide not containing boron. As the alkenylsuccinimide and/or the alkylsuccinimide, there are mentioned the above-mentioned alkenyl or alkylsuccinic monoimide and alkenyl or alkylsuccinic bisimide.

The amount of the boron-free succinimide is not specifically defined, but is generally 0.1 to 10% by mass or so based on the total amount of the composition, more preferably 0.5 to 5% by mass or so.

The lubricating oil composition may further contain an antioxidant. The antioxidant includes an amine-type antioxidant, a phenolic antioxidant, a sulfur-type antioxidant, a phosphorus-type antioxidant, a molybdenum amine complex-type antioxidant, and so on. Among these, an amine-type antioxidant and a phenolic antioxidant are preferred. For these, any one or more may be suitably selected from known antioxidants that are heretofore used as an antioxidant for a lubricating oil.

Examples of the amine-type antioxidant include diphenylamine-type ones such as diphenylamine, a dialkyldiphenylamine where the alkyl group has 3 to 20 carbon atoms, naphthylamine-type ones such as α-naphthylamine, an alkyl-substituted phenyl-α-naphthylamine where the alkyl group has 3 to 20 carbon atoms, etc.

Examples of the phenolic antioxidant include monophenol-type ones such as 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenynpropionate, etc.; diphenol-type ones such as 4,4′-methylenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), etc.

The sulfur-type antioxidant includes dilauryl-3,3′-thiodipropionate, etc.; the phosphorus-type antioxidant includes phosphites, etc.

The molybdenum amine complex-type antioxidant includes 6-valent molybdenum compounds, specifically those produced through reaction of molybdenum trioxide and/or molybdic acid and an amine compound, for example, compounds obtained according to the production method described in JP 2003-252887A.

One alone or two or more kinds of these antioxidants may be used either singly or as combined, but in general, two or more kinds are preferably used as combined.

The content of the antioxidant is preferably 0.01 to 10% by mass or so based on the total amount of the composition, more preferably 0.1 to 5% by mass or so.

Further, the lubricating oil composition may contain at least one additive selected from any other friction modifier and anti-wear agent than those mentioned hereinabove.

Specifically, there are mentioned, for example, sulfur-type compounds such as sulfurized olefins, dialkyl polysulfides, diarylalkyl polysulfides, diaryl polysulfides, etc.; phosphorus-type compounds such as phosphates, thiophosphates, phosphites, alkylhydrogen phosphites, phosphate amine salts, phosphite amine salts, etc.; organic metallic compounds such as zinc dithiocarbamate (ZnDTC), sulfurized oxymolybdenum organophosphorodithioate (MoDTP), sulfurized oxymolybdenum dithiocarbamate (MoDTC), etc.; ashless friction modifiers such as amine compounds, fatty acid esters, fatty acid amides, fatty acids, aliphatic alcohols, aliphatic ethers, urea compounds, hydrazide compounds, etc. One alone or two or more kinds of these may be used either singly or as combined.

Among these, from the viewpoint of energy-saving performance, use of sulfurized oxymolybdenum dithiocarbamate is preferred. The content of these friction modifier and anti-wear agent is preferably 0.01 to 8% by mass or so based on the total amount of the composition, more preferably 0.1 to 5% by mass.

The lubricating oil composition may further contain other component such as a pour point depressant, a metal deactivator, a pour point depressant, a defoaming agent, etc.

The kinematic viscosity at 100° C. of the lubricant oil composition of the present invention is not specifically limited, and is generally 2 to 25 mm²/s or so, preferably 3 to 22 mm²/s, more preferably 4 to 17 mm²/s. Having such a low viscosity, the composition can readily realize fuel-saving performance. The viscosity index of the lubricating oil composition is preferably 150 or more, more preferably 170 to 300 or so, even more preferably 180 to 250 or so.

The lubricating oil composition of the present invention is a lubricating oil composition for an internal combustion engine, which is used in various kinds of internal combustion engines such as those in four-wheeled vehicles, two-wheeled vehicles, etc. For driving in an urban area where stop-and-go driving is repeated, when an engine having a turbo mechanism mounted therein, which can be high-powered, is used for example, a lubricating oil composition used in the internal combustion engine has been often troubled by problems of coking and copper release, but the lubricating oil composition of the present invention can prevent coking and copper release in a well-balanced manner.

[Production Method for Lubricating Oil Composition]

According to the production method for the lubricating oil composition of the present invention, a lubricating oil composition is produced by blending the above-mentioned components (B) and (C) into the lubricant base oil (A). In the production method for the lubricating oil composition of the present invention, the above-mentioned components (D), (E) and/or any other components than the components (B) and (C) may be blended into the lubricant base oil.

The amount of the lubricant base oil (A) and the amounts (namely, the blending amounts) of the above-mentioned components (B) to (E) and other components are the same as the content of each component mentioned above, and the properties of the lubricating oil composition and the details of the constituent components are also the same as those mentioned above, and therefore describing them is omitted here.

In the production method, the components may be blended into the base oil in any mode and the means for the addition is not limited.

The lubricating oil composition, produced by blending the components (B) and (C) and optionally by further blending the components (D) and (E) and any other component than these, generally contains these components that are blended thereto, but as the case may be, at least a part of the blended additives may be converted into any other compound through reaction or the like.

EXAMPLES

Next, the present invention is described in more detail by Examples, but the present invention is not whatsoever limited by these Examples.

In this description, measurement of various physical properties and evaluation of the lubricating oil composition are carried out according to the schemes mentioned below.

(1) Kinematic Viscosity

This is a value measured using a glass-made capillary viscometer according to JIS K2283.

(2) Viscosity Index

This is a value measured according to JIS K2283.

(3) NOACK Value

This is a value measured according to the method defined in JPI-5S-41.

(4) Paraffin Content (% C_(P)) According to Ring Analysis

This indicates a proportion (percentage) of the paraffin component calculated through n-d-M ring analysis, and is measured according to ASTM-D-3238.

(5) Base Number

This is a value measured according to a perchloric acid method according to JIS K2501.

(6) Mean Carbon Number (X) in Poly(meth)acrylate

This is a value calculated by the chemical shift and the integrated value in ¹³C-NMR. Specifically, from the total of the integrated values of the alkyl groups and the integrated value of each alkyl group, the proportion of each alkyl group is firstly calculated, and then the mean carbon number is calculated according to the following formula.

Mean carbon number X=total of (carbon number of each alkyl group×proportion of each alkyl group)

The measurement conditions in ¹³C-NMR are as follows.

Apparatus: ECX-400P (manufactured by JEOL Ltd.)

Solvent: CDCl3

Resonant frequency: 100 MHz Measurement mode: gated decoupling method Integration frequencies: 2,000 to 5,000 Pulse delay time: 25 s Pulse width: 9.25 μs

X-angle: 90° (7) Weight-Average Molecular Weight of Poly(meth)acrylate (Mw)

The weight-average molecular weight (Mw) is measured under the following conditions, and is a value obtained based on a calibration curve of polystyrene. Precisely, the value is measured under the following conditions.

Apparatus: 1260 Model HPLC manufactured by Agilent Technologies, Inc. Columns: Shodex LF 404, two columns Solvent: chloroform

Temperature: 35° C.

Sample concentration: 0.05% Calibration curve: polystyrene Detector: differential refractive index detector (8) Total Base Number and Base Number Reduction after Degradation in ISOT

In an ISOT test (165.5° C.) according to JIS K 2514, a copper piece and an iron piece serving as a catalyst are put into a test oil (lubricating oil composition) and the test oil is forcedly degraded. After 96 hours, the total base number is measured (according to a perchloric acid method). In addition, the decrease ratio of the total base number of the test oil by degradation relative to the total base number of a fresh oil is calculated. The oil whose decrease ratio is lower has a higher base number retention, and is a long-drain oil which is capable of being used for a long period of time.

(9) Copper Release after Degradation in ISOT

The copper release in the test oil after degradation in the above-mentioned ISOT test is measured.

(10) Panel Coking Test

According to a Federal test method 791B-3462, each oil composition is tested under a condition at a panel temperature of 300° C. and an oil temperature of 100° C., and in a cycle of a splash time of 15 seconds and a cessation time of 45 seconds, for 3 hours. After the test, the coked substance adhering to the panel is evaluated.

Examples 1 to 9, Comparative Examples 1 to 4

As shown in Table 1, the components (B) to (E) and other components were blended into the lubricant base oil (A) to produce the lubricating oil compositions of Examples and Comparative Examples each containing the lubricant base oil (A) and these components. These lubricating oil compositions were evaluated, and the results are shown in Table 1.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Formulation of Lubricating Oil Composition Lubricant Base Oil (A1) mass % balance balance balance balance balance — balance Lubricant Base Oil (A2) mass % — — — — — balance — Lubricant Base Oil (A3) mass % — — — — — — — Lubricant Base Oil (A4) mass % — — — — — — — Boron-containing Succinimide (B1) mass % 2.00 2.00 2.00 2.00 2.00 2.00 3.00 Poly(meth)acrylate (C1) mass % 15.90 — — — — — 15.50 Poly(meth)acrylate (C2) mass % — 12.00 — — — 9.20 — Poly(meth)acrylate (C3) mass % — — 20.00 — — — — Poly(meth)acrylate (C4) mass % — — — 12.77 — — — Poly(meth)acrylate (C5) mass % — — — — 5.45 — — Poly(meth)acrylate (C6) mass % — — — — — — — Poly(meth)acrylate (C7) mass % — — — — — — — Poly(meth)acrylate (C8) mass % — — — — — — — ZnDTP (D1) mass % 1.20 1.20 1.20 1.20 1.20 0.90 1.20 Metallic Detergent (E1) mass % 0.80 0.80 0.80 0.80 0.80 — 0.80 Metallic Detergent (E2) mass % 0.80 0.80 0.80 0.80 0.80 2.90 0.80 Metallic Detergent (E3) mass % 0.80 0.80 0.80 0.80 0.80 — 0.80 Boron-free Succinimide mass % 1.80 1.80 1.80 1.80 1.80 1.80 1.80 Amine-type Antioxidant mass % 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Phenolic Antioxidant mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 MoDTC mass % — — — — — 0.07 — Content of Each Component Boron-equivalent amount or mass % 0.026 0.026 0.026 0.026 0.026 0.026 0.039 component (B1) Resin content in component (C) mass % 4.45 2.28 3.20 3.32 2.29 1.75 4.34 Ca-equivalent amount of mass % 0.23 0.23 0.23 0.23 0.23 0.24 0.23 metallic detergent P-equivalent amount of mass % 0.11 0.11 0.11 0.11 0.11 0.08 0.11 component (D1) Poly(meth)acrylate Mw 200,000 510,000 440,000 370,000 430,000 510,000 200,000 X 4.6 5.7 5.8 5.6 6.3 5.7 4.6 Mw/X 43,478 89,474 75,862 66,071 68,254 89,474 43,478 Evaluation Results of Lubricating Oil Composition Kinematic Viscosity (40° C.) mm²/s 60.2 58.1 60.6 51.1 56.9 31.9 60.1 Kinematic Viscosity (100° C.) mm²/s 12.4 11.9 13.0 12.0 11.9 7.9 12.3 Viscosity Index 211 207 220 240 212 232 208 Panel Coking Test mg 80 40 36 84 88 80 67 Base number of Fresh Oil mgKOH/g 7.7 7.6 7.6 7.6 7.6 9.2 7.8 (perchloric acid method) Base number after mgKOH/g 4.7 4.7 5.1 4.7 4.4 5.8 5.0 degradation in ISOT Base number Reduction Ratio % 38 39 33 37 42 37 36 Copper Release after ppm 54 78 82 37 37 95 41 degradation in ISOT Comparative Comparative Comparative Comparative Example 8 Example 9 Example 1 Example 2 Example 3 Example 4 Formulation of Lubricating Oil Composition Lubricant Base Oil (A1) mass % — — balance balance balance balance Lubricant Base Oil (A2) mass % — balance — — — — Lubricant Base Oil (A3) mass % balance — — — — — Lubricant Base Oil (A4) mass % 5.00 — — — — — Boron-containing Succinimide (B1) mass % 2.00 2.00 2.00 2.00 2.00 — Poly(meth)acrylate (C1) mass % — — — — — 16.70 Poly(meth)acrylate (C2) mass % 11.50 9.20 — — — — Poly(meth)acrylate (C3) mass % — — — — — — Poly(meth)acrylate (C4) mass % — — — — — — Poly(meth)acrylate (C5) mass % — — — — — — Poly(meth)acrylate (C6) mass % — — 12.30 — — — Poly(meth)acrylate (C7) mass % — — — 7.80 — — Poly(meth)acrylate (C8) mass % — — — — 5.50 — ZnDTP (D1) mass % 0.90 0.90 1.20 1.20 1.20 1.20 Metallic Detergent (E1) mass % — — 0.80 0.80 0.80 0.80 Metallic Detergent (E2) mass % 2.90 1.50 0.80 0.80 0.80 0.80 Metallic Detergent (E3) mass % — — 0.80 0.80 0.80 0.80 Boron-free Succinimide mass % 1.80 1.80 1.80 1.80 1.80 1.80 Amine-type Antioxidant mass % 1.00 1.00 1.00 1.00 1.00 1.00 Phenolic Antioxidant mass % 0.50 0.50 0.50 0.50 0.50 0.50 MoDTC mass % 0.07 0.07 — — — — Content of Each Component Boron-equivalent amount or mass % 0.026 0.026 0.026 0.026 0.026 — component (B1) Resin content in component (C) mass % 2.19 1.75 6.52 3.59 2.42 4.68 Ca-equivalent amount of mass % 0.24 0.12 0.23 0.23 0.23 0.23 metallic detergent P-equivalent amount of mass % 0.08 0.08 0.11 0.11 0.11 0.11 component (D1) Poly(meth)acrylate Mw 510,000 510,000 44,000 90,000 210,000 200,000 X 5.7 5.7 7.3 8.1 9.4 4.6 Mw/X 89,474 89,474 6,027 11,111 22,340 43,478 Evaluation Results of Lubricating Oil Composition Kinematic Viscosity (40° C.) mm²/s 32.2 32.0 76.2 69.0 67.1 60.4 Kinematic Viscosity (100° C.) mm²/s 8.2 7.9 12.8 12.4 12.3 12.6 Viscosity Index 245 232 170 180 184 213 Panel Coking Test mg 69 86 166 169 77 157 Base number of Fresh Oil mgKOH/g 9.0 6.5 7.7 8.2 8.0 7.4 (perchloric acid method) Base number after mgKOH/g 6.2 3.7 5.0 3.1 2.6 4.5 degradation in ISOT Base number Reduction Ratio % 31 43 34 62 68 39 Copper Release after ppm 87 101 167 120 286 115 degradation in ISOT * The components in Table 1 are as follows.

(A) Lubricant Base Oil

Lubricant base oil (A1): Group III 150 N hydrorefined base oil, 100° C. kinematic viscosity 6.4 mm²/s, viscosity index 131, NOACK value (250° C., 1 hour) 7.0% by mass, n-d-M ring analysis % C_(P), 79.1% Lubricant base oil (A2): Group III 100 N hydrorefined base oil, 100° C. kinematic viscosity 4.1 mm²/s, viscosity index 134, NOACK value (250° C., 1 hour) 12.9% by mass, n-d-M ring analysis % C_(P), 87.7% Lubricant base oil (A3): Group IV poly-alpha-olefin, 100° C. kinematic viscosity 3.7 mm²/s, viscosity index 117, NOACK value (250° C., 1 hour) 15.6% by mass Lubricant base oil (A4): Group IV ester base oil, 100° C. kinematic viscosity 4.3 mm²/s, viscosity index 139, NOACK value (250° C., 1 hour) 2.6% by mass (In Example 8, the lubricant base oil was a mixture of the lubricant base oil (A3) and the lubricant base oil (A4), and the kinematic viscosity at 100° C. of the mixed base oil was 4.3 mm²/s and the viscosity index thereof was 130.)

(B) Boron-Containing Succinimide

Boron-containing succinimide (B1); polybutenylsuccinimide boride, boron content 1.3% by mass, nitrogen content 1.2% by mass, weight-average molecular weight of polybutenyl group 1,800, B/N ratio 1.1

(C) Poly(meth)acrylate

Poly(meth)acrylate (C1): polyalkyl (meth)acrylate, weight-average molecular weight 200,000, mean carbon number (X) 4.6, resin content 28% by mass Poly(meth)acrylate (C2): polyalkyl (meth)acrylate, weight-average molecular weight 510,000, mean carbon number (X) 5.7, resin content 19% by mass Poly(meth)acrylate (C3): polyalkyl (meth)acrylate, weight-average molecular weight 440,000, mean carbon number (X) 5.8, resin content 16% by mass Poly(meth)acrylate (C4): polyalkyl (meth)acrylate, weight-average molecular weight 370,000, mean carbon number (X) 5.6, resin content 26% by mass Poly(meth)acrylate (C5): polyalkyl (meth)acrylate, weight-average molecular weight 430,000, mean carbon number (X) 6.3, resin content 42% by mass Poly(meth)acrylate (C6): polyalkyl (meth)acrylate, weight-average molecular weight 44,000, mean carbon number (X) 7.3, resin content 53% by mass Poly(meth)acrylate (C7): polyalkyl (meth)acrylate, weight-average molecular weight 90,000, mean carbon number (X) 8.1, resin content 46% by mass Poly(meth)acrylate (C8): polyalkyl (meth)acrylate, weight-average molecular weight 210,000, mean carbon number (X) 9.4, resin content 44% by mass

(D) Zinc Dithiophosphate

ZnDTP (D1): zinc dialkyldithiophosphate, zinc content 9.0% by mass, phosphorus content 8.2% by mass, sulfur content 17.1% by mass, alkyl group: mix of secondary butyl group and secondary hexyl group

(E) Metallic Detergent

Metallic detergent (E1): basic calcium phenate, total base number (perchloric acid method) 255 mgKOH/g, calcium content 9.3% by mass, sulfur content 3.0% by mass Metallic detergent (E2): basic calcium salicylate, total base number (perchloric acid method) 225 mgKOH/g, calcium content 7.8% by mass, sulfur content 0.2% by mass Metallic detergent (E3): basic calcium sulfonate, total base number (perchloric acid method) 300 mgKOH/g, calcium content 11.6% by mass, sulfur content 1.49% by mass

Other Components

Boron-free succinimide: polybutenylsuccinic bisimide, number-average molecular weight of polybutenyl group 2300, nitrogen content 1.0% by mass, chlorine content 0.01% by mass or less Amine-type antioxidant: dialkyldiphenylamine, nitrogen content 4.62% by mass Phenolic antioxidant: octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate MoDTC: sulfurized oxymolybdenum dithiocarbamate, molybdenum content 10.0% by mass, sulfur content 11.5% by mass

As obvious from the results in Table 1, the lubricating oil compositions of Examples 1 to 9 each contained a boron-containing succinimide and a polyalkyl (meth)acrylate having a specific Mw and a specific Mw/X, and therefore could suppress coking occurrence and copper release in the degradation test while preventing base number reduction therein.

On the other hand, in Comparative Examples 1 to 3, Mw and Mw/X of the polyalkyl (meth)acrylate did not fall within a predetermined range, and therefore coking occurrence and copper release could not be prevented sufficiently. The lubricating oil composition of Comparative Example 4 did not contain a boron-containing succinimide, and therefore, even though Mw and Mw/X of the polyalkyl (meth)acrylate therein each fell within a predetermined range, coking occurrence and copper release could not be prevented sufficiently.

INDUSTRIAL APPLICABILITY

The lubricating oil composition for an external combustion engine of the present invention can prevent base number reduction, coking occurrence and copper release in a well-balanced manner, and therefore can be favorably used, for example, in an internal combustion engine for automobiles. 

1. A lubricating oil composition for an internal combustion engine, the composition comprising: (A) a lubricant base oil comprising a mineral oil, a synthetic oil, or both; (B) a boron-containing alkenylsuccinimide, a boron-containing alkylsuccinimide, or both, in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on a total amount of the composition; and (C) a poly(meth)acrylate in an amount of 0.1 to 30% by mass based on the total amount of the composition, the poly(meth)acrylate having Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, wherein a weight-average molecular weight thereof is represented by Mw, and a mean carbon number of alkyl groups therein, as measured through ¹³C-NMR, is represented by X.
 2. The lubricating oil composition for an internal combustion engine according to claim 1, wherein Mw/X is 30,000 to 200,000.
 3. The lubricating oil composition for an internal combustion engine according to claim 1, wherein the poly(meth)acrylate (C) is a non-dispersive poly(meth)acrylate.
 4. The lubricating oil composition for an internal combustion engine according to claim 1, wherein a viscosity index of the lubricant base oil (A) is 90 or more.
 5. The lubricating oil composition for an internal combustion engine according to claim 1, comprising a mineral oil having a paraffin content (% C_(P)) according to ring analysis of 60% or more.
 6. The lubricating oil composition for an internal combustion engine according to claim 1, further comprising at least one selected from the group consisting of (D) a zinc dithiophosphate, and (E) an alkali metal detergent or an alkaline earth metal detergent.
 7. The lubricating oil composition for an internal combustion engine according to claim 6, wherein the composition comprises the zinc dithiophosphate (D) in an amount of 0.01 to 0.15% by mass as a phosphorus-equivalent amount, and the alkali metal detergent or the alkaline earth metal detergent (E) in an amount of 0.1 to 0.3% by mass as a metal-equivalent amount, based on the total amount of the composition.
 8. The lubricating oil composition for an internal combustion engine according to claim 1, wherein the composition has a kinematic viscosity at 100° C. of 4 to 17 mm²/s.
 9. A method of producing a lubricating oil composition for an internal combustion engine, the method comprising blending (B) a boron-containing alkenylsuccinimide, a boron-containing alkylsuccinimide, or both, in an amount of 0.001 to 0.1% by mass as a boron-equivalent amount based on the total amount of the composition, and (C) a poly(meth)acrylate in an amount of 0.1 to 30% by mass based on the total amount of the composition, into (A) a lubricant base oil comprising a mineral oil, a synthetic oil, or both, wherein the poly(meth)acrylate (C) has a Mw of 100,000 to 700,000 and Mw/X of 30,000 or more, in which a weight-average molecular weight thereof is represented by Mw and a mean carbon number of alkyl groups therein, as measured through ¹³C-NMR, is represented by X. 